1. Field of the Invention
The present invention relates to the field of wave meters for accurately determining the wavelength of light, such as the wavelength of the emission or a component of the emission of a laser.
2. Prior Art
It may be desired in various applications to accurately measure the wavelength of light (within or without the visible spectrum) or a component part thereof so that such wavelengths will be accurately known. In some applications, it is desired to measure the wavelength of the light to determine its variance from a predetermined desired wavelength, either to determine the error in the wavelength or to provide an error signal, preferably a proportional error signal, which may be used to correct the deviation from the desired wavelength. Since the drift in the setting of a tunable laser will be excessive in applications requiring precise wavelength setting and control, it is preferable to provide a high accuracy measurement of the wavelength of the emission of the laser and to use the same in a closed loop system to null the error, providing wavelength control of the laser emission substantially within the accuracy of the wavelength measurement.
A system for and method of regulating the wavelength of a light beam such as in a tunable laser is disclosed in U.S. Pat. No. 5,025,445. The system disclosed therein splits out approximately 5% of the laser emission and provides both a coarse and fine wavelength measurement thereof to provide a real-time correction of the wavelength of the laser emission. The fine measurement provides high accuracy within a narrow wavelength band, though in essence repeats in corresponding adjacent narrow wavelength bands so as to provide a possible ambiguity in the true wavelength being measured. The coarse measurement, on the other hand, spans the bands of ambiguity to resolve the same, allowing the accurate measurement of the wavelength of the laser emission without ambiguity.
It is stated in the '445 patent that the system disclosed therein which includes an etalon may be disposed in an evacuated housing or a housing filled with a suitably inert material such as nitrogen, and that the same "is not affected by atmospheric changes in temperature and pressure." The extent to which the statement is true, of course, depends upon the degree of accuracy desired. For the most demanding applications such as, by way of example, fixed wavelength deep ultraviolet light sources (excimer lasers) for photolithographic semiconductor integrated circuit fabrication processes utilizing the latest sub-micron line widths, extreme stability in the ultraviolet wavelength is desired. In such applications, temperature sensitivity of the wave meter in general and of the etalon in particular needs to be eliminated to the maximum extent possible.
As stated in the '445 patent, the etalon assembly may be disposed in a housing "filled" with a vacuum, in which case, in theory, the index of refraction between the reflecting surfaces of the etalon will be and remain constant at the index of refraction of free space (1.0). However, a good vacuum is difficult to maintain. A high quality vacuum requires an enclosure which may be pumped down to a very high level of evacuation and then hermetically sealed. The vacuum requirement also limits the nature of the materials which may be used within the enclosure to those having low outgassing characteristics (low moisture and other gaseous absorption and/or inherent volatile component content). It also indirectly limits physical configurations, in that air spaces having limited communication to the vacuum region will pump down very slowly, or alternatively continue to bleed gas into the vacuum region long after one believes a good vacuum is achieved and the enclosure has been sealed off. By way of specific example, screws holding parts together within a vacuum enclosure can trap a substantial amount of gas in the bottom of the hole into which the screw is threaded, within the thread region and within the screw shank clearance hole or holes of the parts retained by the screws. This gas may take days or even weeks to leak out into the main evacuated area and particularly between the etalon mirrors, to cause an increase in the index of refraction of the now partly gaseous filled space between the etalon mirrors, causing a deviation in the wavelength sensing thereof. Finally, providing an enclosure capable of maintaining a relatively hard vacuum and obtaining the same is expensive, and makes the product difficult to service at the factory and essentially non-field serviceable.
Also as mentioned in the '445 patent, the etalon assembly may alternatively be filled with an inert gas such as nitrogen. This has the advantage of eliminating any of the problems associated with obtaining and maintaining a relative hard vacuum. However, since the volume of the enclosure containing the nitrogen changes somewhat with temperature due to the thermal coefficient of expansion of the enclosure materials, the density of the nitrogen in the enclosure will be dependent on temperature, resulting in a variable index of refraction of the gas between the etalon mirrors dependent on the temperature at which the etalon and enclosure stabilizes.
Since the etalon output will drift with temperature more than can be tolerated in the most exacting applications, etalon stability has been enhanced in the prior art by effectively placing the etalon or entire wave meter in a small oven-like structure and elevating the temperature thereof to a predetermined stable temperature which may be maintained by an appropriate controller irrespective of changes in ambient temperature. Such temperature control can substantially enhance the accuracy of the wave meter once the desired temperature has been achieved and the system including the temperature controller stabilized. However, this requires a substantial amount of time from a "cold" start of the laser system before the desired accuracy and stability in the wavelength measurements are obtained. To reduce the startup time, the etalon and/or the entire wave meter heating system may be left on and thus stabilized at the elevated temperature even when the laser is turned off. Aside from the inconvenience and the waste of power in doing this, the reduction in start-up time achieved is limited because of the change in heat load on the oven due to the step change in heat given off by the rest of the laser system when the same is turned on. In particular, on/off type controllers would provide continuous and unacceptable thermal transients to the etalon and the rest of the wave meter, resulting in cyclic and excessive inaccuracies thereof. More modern controllers stabilize to a substantially uniform power output based upon the heat load (dissipation) of the heated portion of the system to maintain a substantially constant temperature under stable quiescent conditions, but will also provide excessive temperature excursions around the desired temperature when the laser system is first turned on until there is an ample opportunity for the system to re-stabilize at the new heat load. Consequently the always on oven approach also exhibits transient characteristics much like the warm-up requirement when the laser system and wave meter is first turned on during a cold startup.